Typically, a resonant antenna is designed to operate when it is a half wavelength long, although in some instances a quarter wavelength design is sufficient. Antennas that are designed to operate at a tenth of a wavelength or less are typically termed electrically small. Most electrically small antennas (ESA) exhibit high impedance mismatch and low efficiency. Furthermore, the few ESA designs which have been developed to date are inherently very narrowband, due to the limited volume that these ESAs occupy.
Electrically small antennas exhibit poor efficiency because their driving point impedance is inherently quite capacitive. Typically, antenna developers electrically enlarge these antennas by transforming a small dipole into a lengthy coil and thereby create a large inductance to cancel the capacitive reactance of the electrically short dipole. Furthermore, two and four arm folded spherical helixes can be used to increase the very low driving point resistance of the antenna, so as to match to the characteristic impedance of the feeding transmission line to allow for efficient radiation of the ESA; such antennas remain extremely narrow band.